At present, spatial measurement of beam bundles is mostly carried out by means of optoelectronic sensors such as, e.g., individual or arrayed photodiodes, CCD detectors and CMOS detectors, or cameras arranged behind fluorescent screens, or photon counters. In so doing, the radiation impinging on the sensor as input signal produces an electric output signal; the functional relationship between the input signal strength and output signal strength is described by a characteristic curve specific to each detector and type of radiation (particle type, wavelength).
Characteristic curves of optoelectronic sensors have a linear relationship between the input signal and output signal, typically over a range suitable for technical measurement purposes. Depending on the way in which the sensors operate, the characteristic passes after a certain value of the input signal into a saturation state which is not suitable for measuring purposes.
As increasingly powerful radiation sources are developed, measurement of radiation quantities in short-wavelength ionizing radiation, even at powers of >100 W, require much more robust sensors which must nevertheless be sufficiently sensitive to slight power fluctuations.
When sensors are used for measuring purposes of this kind, e.g., for soft X-ray radiation (EUV), the high radiation energies and intensities quickly lead to saturation or even destruction, e.g., due to high heat development in the sensors. The use of optical attenuators such as absorption filters between the radiation source and the sensor is also possible only to a limited extent because of high heat development.
For these reasons, intensities within a beam bundle of high-energy radiation such as UV, DUV, EUV, X-ray, or laser radiation are usually measured indirectly.
U.S. Pat. No. 7,023,524 B2, for example, describes the arrangement of movable apertures for deliberate patterning of the beam in or near an intermediate focus in the beam path. Edge beams, known as “out-of-field” beams, are acquired by a photodiode and the intensity of the radiation is determined therefrom. However, only an equivalent of the total intensity of the source, and not a distribution of intensities over the cross section of the beam bundle, is determined by this variant of radiation measurement, since the determined value does not originate from the region of the beam bundle used for machining. In order to overcome this deficiency an indirect method for measuring the intensity distribution over the cross section of the beam bundle is proposed, wherein the electrical resistance or other electrical parameters which vary as a result of the heating during irradiation are measured at the moving apertures, and an intensity profile is derived therefrom. However, this solution achieves only a spatial resolution in the dimension of the size of the apertures.
In order to take measurements inside the beam bundle and to carry this out while machining of a workpiece is in progress, DE 82 27 494 U1 discloses an arrangement in which only a small fraction of the laser cross section is coupled out stripewise to a measuring device through reflection so that measurement is possible during laser cutting of a workpiece. To this end, a reflecting round rod is moved on a circular path through the beam path. Since the relative position of the reflector in relation to the beam bundle is known at all times, the respective measurement values obtained can be spatially associated with a stripe-shaped section of the beam bundle.
However, these measurement data of the beam bundle by way of the respective stripes are averaged values and therefore cannot yield intensity values with high point-by-point resolution.